1. Field of the Invention
The present invention is generally related to power amplifiers, and, more specifically, to a system and method for regulating the power produced by a power amplifier.
2. Discussion of the Background
In certain communication systems, for example, the Global System for Mobile Communication (GSM), the output power of a transmitter within a communication device (e.g., a mobile phone) has to be well controlled due to the specifications of the communication system. In transmitters used in GSM and GSM like systems, the output power of the transmitter is generated by a power amplifier, which is controlled by a control circuit.
The output power (P) of a power amplifier is a function of load impedance (R), as shown by the following formula: P=I2R, where I is the current provided to the load from the power amplifier. Thus, if the load impedance (R) changes, the output power (P) will change (assuming I is kept constant). Changes to the load impedance are commonly caused by environmental factors. For example, if the load is an antenna of a transmitter for transmitting signals, the load impedance is dependent upon the environment surrounding the antenna. As an example, if a mobile phone is held in the hand, the impedance “felt” by the power amplifier may be different from the impedance felt by the power amplifier when the mobile phone is sitting on a table or in a car. Because the output power must be well controlled, it is desired to keep the output power constant, regardless of changes in the load impedance.
Conventionally, the output power has been controlled though the use of a current detector. FIG. 1 illustrates a conventional circuit for controlling the output power of a power amplifier 102. As shown in FIG. 1, the current (I1) flowing into power amplifier 102 is measured. I1 is a good measure of the power produced by power amplifier 102. This current is measured by measuring the voltage at node N1. This voltage is referred to as Vsense. By Ohm's law, Vsense is related to (I1) as follows: Vsense=Vbat−(I1)R1, where Vbat is the voltage produced by power source 110. L1 is an RF-choke inductor. A control circuit 108 compares Vsense to a reference voltage (Vref). Control circuit 108 controls the output power of power amplifier 102 based on the result of the comparison between Vsense and Vref. In this manner, the circuit shown in FIG. 1 may control the output power of power amplifier 102 based on the measured current (I1) flowing into the power amplifier 102. The circuit shown in FIG. 1 suffers from several disadvantages, one of which is that it has difficulty achieving high dynamics.
What is desired, therefore, is a system and/or method to improve the power regulation of a power amplifier circuit.